hi shona,
you don't have to consciously roll to the heel, it just kinda happens, or not. it's all about the initial point of impact being something other than the boney heel of the foot.
if you do run in the minimalists, then you'll probably find that you have no choice than not to heel strike as heel striking in thin soled shoes is too uncomfortable.
if you heel strike now and want to move to fore / mid foot then the calves will be doing something new and different parts of the calf muscle come into play. they need to be broken in gently and warmed up before running, as does the achilles. with heel striking, the force acts pretty much directly on the bones & joints. with fore / mid foot, the muscles and tendons are acting as the shock absorbers. this is something they've not been used to doing. there are good resources on the merrel and vibram sites for transitioning to minimal shoes / fore foot. to start with, it's recommended to not try running any further than 1/4 mile and to gradually build up the distances and to add in lots of other stuff to help your feet with the transition.
i was the classic too far, too soon and kept pinging the calves (october last year). it only put my out for 7 then 4 then 3 days on the 3 occassions when they cried "enough!", however i'm now running 5 miles comfortably (on tarmac) with training runs of 3 miles and dreadmill recovery runs of 1 mile.
iirc, i was recommended 1 long and 1 short run per week to start with, interspersed with calf raise strengthening excercises. given you're starting at 1/4 mile max, the short run becomes very short!
as before, i'm no expert in this and the above is based on my experience of running forefoot in minimalist daps.
let us know how you get on with the minimalist shoes. if you're anything like me, they'll feel well weird to start with, but you soon love them as the feel & feedback you get through the feet is awesome! wearing cushioned shoes then becomes well strange as all the feedback you were getting suddenly gets muted.